Single-phase brushless DC motors are extensively used in the manufacturing the CPU cooler fan because it has the advantage of a lowered fabrication cost due to the easier assembly process and the higher fabrication reliability. However, the main concern over the motor design turns to how to lower the cogging torque and prevent the motor from being blocked on the dead initiating position when the motor of such kind is adapted for use under the conditions of a required low cogging torque (for example, the spindle motor in a CD-ROM). The lower cogging torque will require a greater precision in the assembly process. On the contrary, the higher cogging torque will induce the larger vibration in rotation to inevitably reduce the maneuverability of the motor. The problem that the rotor of a motor can be retained at the dead initiating position and thus become not easy to be re-started after the stop of the motor is therefore taken into account. The measure as known to the art is to resolve the problem by adopting the concept of detouring the magnetic flux and there are two ways of the most adopted. The first of which is to trim every pole of the stator into the asymmetrical shape while leaving the polar plates from trimming (as disclosed in the U.S. Pat. No. 5,093,599, U.S. Pat. No. 4,987,331, and U.S. Pat. No. 5,492,458). The cogging torque curve 16 as formed by such way is as illustrated in the FIG. 1. The rotor will stay at the point C as referred to FIG. 1 when the motor stops. By that, the problem caused by the dead initiating position will be avoided; however, the cogging torque for the motor is also increased. The second way is to keep the shapes of the poles of the stator in symmetric while trimming the edges of the polar plates into the arcs (as disclosed in the U.S. Pat. No. 5,744,889). The stopping position of the rotor will appear in two conditions as illustrated in FIG. 1. (1) When the stator is overly trimmed or inaccurately assembled, the cogging torque of the motor can probably turn to be inverse (as referred to the inverse curve 17 in FIG. 1). The stopping position of the rotor can be displaced to the dead initiating position A or E, which will cause to a difficult start for the motor. (2) Even if the polar plates of the motor are properly trimmed to lead the rotor to stop at point B or point D, the actual stop position of the rotor can be possibly close to or even exactly located on the dead initiating position A or E due to the mechanical friction, which has great influences on the manufacturing reliability in the mass-production. The cogging torque curve 18 for the motor assembly with the trimmed stator is as shown in FIG. 1, which presents a substantial shift on the stopping position compared to the motor with an untrimmed stator. A severe demand on the exact tolerances for the parts and the high accuracy on the assembly process should be therefore adopted to avoid the occurrence of such problems. From that, it can be concluded that the disclosure as known to the art cannot practically resolve the problems on the difficult start of the motor simply by detouring the magnetic flux to prevent the rotor from staying at the dead initiating position when the motor stops. Moreover, additional problems like the severe demand on the quality of the products and high accuracy on the assembly procedure are still raising to reduce the manufacturing reliability in the mass-production.
In the two disclosures as known to the art for resolving the problems caused by the dead initiating position in the single-phase DC brushless motor, the additional problems like the increasing of the cogging torque or the increasing probability on the rotor's stopping at the dead initiating position will be still rising no matter the shape of the stator is trimmed to an asymmetrical one or the shape of the stator is kept symmetrical while the edges of the polar plates are trimmed into arcs and yet the problems cannot be resolved.